{
  "name": "sand_falls_down",
  "description": "Sand placed in empty space should fall due to gravity",
  "seed": 42,
  "init_mode": "empty",
  "setup": [
    {"type": "fill_rect", "x": 100, "y": 110, "w": 10, "h": 1, "material": "stone"},
    {"type": "set_cell", "x": 105, "y": 105, "material": "sand"}
  ],
  "steps": 10,
  "assertions": [
    {"type": "cell_is", "x": 105, "y": 109, "material": "sand"},
    {"type": "cell_is_not", "x": 105, "y": 105, "material": "sand"}
  ]
}